A number of assay devices for use in the measurement of one or more analytes in a liquid sample are known. Some of these are complex devices for use by trained personnel; for example, sampling devices for industrial liquids, water etc. or laboratory tests on blood, urine and the like. However, an important category of assay devices are intended for home use by consumers. It is important that assay devices are as simple to operate and interpret as possible, particularly when intended for home use, and it is also important that users have confidence in the reliability of the results.
Examples of such home monitoring assay devices include pregnancy test kits, for example, the Clearblue test from Unipath, England (Clearblue is a registered Trade Mark) and as described in EP 291194 and EP 560411. Home pregnancy tests typically determine the amount of the hormone human chorionic gonadotrophin (hCG) in a urine sample, and normally provide a visual indication as to whether a subject is pregnant.
Some assay devices, such as the Clearblue® test, are self-contained disposable devices. Other disposable assay devices, usually containing one or more reagents to react with an analyte in a urine sample, are used in conjunction with other devices, for example non-disposable electronic meters. This category of product includes the Persona® monitor, from Unipath, which indicates fertile and non-fertile days in the menstrual cycle, and the Clearplan® test kit, also from Unipath, which identifies the most likely time of ovulation in a cycle. Both of these products monitor hormone concentrations in daily urine samples and comprise electronic measuring and/or recording devices and separate, disposable, assay devices for taking an individual urine sample and carrying out the relevant assay.
These assay devices have wicks for taking up a urine sample and delivering it to a reaction zone where analyte of interest in the sample (if any) reacts with one or more reagents, such as labelled antibodies.
When using a wick to take up a urine sample, it is often hard to see that a sample has been taken up correctly or that sufficient sample has been applied to the wick. More specifically, with this type of device, the urine sample is often obtained directly from the urine stream and so it is difficult for the user to see or control the sampling operation and therefore have confidence that the sampling operation has been performed correctly. It would be advantageous to mitigate this potential concern and reassure users that the device has been correctly operated.
U.S. Pat. No. 3,811,840 discloses a “dip-and-read” type assay device comprising a bibulous wick for immersion in a liquid sample. The wick may comprise a chemical indicator which gives a signal after a predetermined time “to give a visual indication of saturation of the wick to the extent desired”. The document teaches the importance of placing the chemical indicator downstream of the detection zone, so as to ensure that the user is instructed to remove the assay device from the sample only once sufficient saturation of the wick has taken place. Thus, there is a time delay between contacting the assay device with the liquid sample and the appearance of the visual indication. For example, in the embodiment illustrated in FIG. 8 of U.S. Pat. No. 3,811,840, a fluid sensitive chemical is deposited downsteam of the detection zone.
Any alteration made to conventional assay devices should preferably fulfil commercial criteria of cost and user acceptance, ready disposability, safety and long term stability, and should not compromise other characteristics of the assay devices.